Immigration reform will pass the Senate before the Fourth of July, Senate majority leader Harry Reid (D) of Nevada has vowed. During the two weeks of Senate debate, the fate of President Obama's top second-term priority hangs in the balance. The bipartisan "Gang of Eight" senators that has driven the debate wants an overwhelming Senate vote to both increase pressure on the GOP-led House to move on immigration and to strengthen the Senate's hand in an eventual negotiations over a final immigration bill.

Here are five key senators (or groups of senators) that will be pivotal in the debate.

1.
Marco Rubio (R) of Florida

Sen. Marco Rubio (R) of Florida speaks at a news conference on immigration in Las Vegas earlier this year.

John Locher/Las Vegas Review-Journal/AP/File

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Will he stay or will he go? The Florida Republican was a pivotal piece of the Senate Gang of Eight. His status as a GOP 2016 presidential contender with tea party chops and a powerful personal story about the American immigrant experience has made him the bill's key emissary to the GOP base and conservative lawmakers alike.

Each of the gang’s other Republican members – Sens. Jeff Flake and John McCain of Arizona and Sen. Lindsey Graham of South Carolina – have long been advocates of comprehensive immigration reform. Senator Rubio’s addition to the club appeals to the most energetic wing of the party, the conservative grass-roots that powered Rubio to office in 2010 and keep him among the brightest stars in the Republican presidential firmament.

Rubio has been willing to go where those like Senator McCain, the party “maverick” at odds with the GOP base, cannot: conservative television and talk radio, turf historically inhospitable (to put it mildly) to the immigration reform.

But Rubio is requiring changes for his continued support of the bill.

First up is not attaching potentially-explosive social issues to the measure, which means Sen. Pat Leahy (D) of Vermont’s amendment to allow gay marriages to be recognized in immigration cases is a deal-killer.

Second, he wants to make those who are legalized by the legislation wait longer for certain benefits (such as subsidies under the national health-insurance exchanges established by Obamacare) or he – and Sen. Orrin Hatch (R) of Utah, whose amendments on this subject Rubio has endorsed – will walk.

Third, Rubio has said the bill’s border-security requirements must be strengthened for it to pass the Senate, much less the GOP-controlled House.

While a solution to the first two matters appears in hand, the question of what counts as appropriate border security is vexing to lawmakers on both sides of the aisle. Democrats don’t want to lay insurmountable hurdles on the pathway to citizenship but want to get to enough Republican support to have 70 votes or more for the bill, strengthening their hand in negotiating an eventual compromise bill with the House.

Republicans searching for improved border-security measures want to compel the executive branch to follow through without being seen as trying to kill the immigration-reform process, something many Republicans believe could be a deadly to the party’s chances of reclaiming the White House in 2016 and beyond.

The immigration-reform effort likely would not have come as far and as fast as it has without Rubio. He will be a pivotal arbiter of the border-security question in the weeks to come.

But will he eventually help push it across the finish line or, if his demands aren’t met, sink it altogether?